Isara mo ang kurtina kung mainit sa hapon.

Isara is an imperative form, so it gives a command: close.

More specifically, it is the pattern commonly used when the thing being acted on is the noun marked by ang. In this sentence, that thing is ang kurtina, the thing being closed.

So isara mo ang kurtina is the natural way to say close the curtain.

Mo is the short pronoun meaning you in this sentence.

In Filipino, short pronouns like mo, ko, ka, niya often come right after the first word of the clause. That is why you get:

Isara mo ang kurtina

not normally Mo isara ang kurtina.

So the order here is very typical.

This is one of the biggest differences from English.

In Filipino, ang does not simply mean the. It marks the noun that is in focus/topic in the sentence. With isara, the thing being closed is the focused noun, so it takes ang:

ang kurtina

Even though English would think of the curtain as the direct object, Filipino marks it with ang because of the verb pattern being used.

Because Filipino does not need a verb like English to be in this kind of sentence.

Mainit already means hot, and it can function as the predicate by itself. So:

kung mainit sa hapon

is literally something like if hot in the afternoon.

English needs it is, but Filipino does not.

Here, kung means if. It introduces a condition:

kung mainit sa hapon
= if it is hot in the afternoon

This is the normal word to use for a basic condition.

Both can relate to conditions, but they are not exactly the same.

• Kung is the basic if
• Kapag often feels more like when or whenever, especially for expected or repeated situations

So kung fits well when you mean if it happens to be hot. In everyday speech, there can be overlap, but kung is the straightforward choice here.

Sa hapon means in the afternoon or during the afternoon.

• sa is a marker often used for location, direction, or time
• hapon means afternoon

In this sentence, sa hapon tells you the time connected with mainit:

kung mainit sa hapon
= if it is hot in the afternoon

Normally, it is understood with mainit:

kung mainit sa hapon
= if it is hot in the afternoon

So the idea is not mainly close it in the afternoon, but rather close it if afternoons are hot.

Context can sometimes affect how people interpret timing, but the most natural reading here is that sa hapon belongs with the condition clause.

Yes. That is completely natural.

Filipino word order is more flexible than English, especially with condition clauses. You can put the kung clause first or last:

• Isara mo ang kurtina kung mainit sa hapon.
• Kung mainit sa hapon, isara mo ang kurtina.

Both mean the same thing. The second version may feel slightly easier to English speakers because the condition comes first.

It is fairly direct. It sounds like a normal command to someone you are familiar with, such as a family member, friend, or child.

If you want to sound more polite, you can add po:

Isara mo po ang kurtina.

A softer request would be:

Pakisara po ang kurtina.

So the original sentence is natural, but it is not especially soft or formal.

Isara and sarhan are related, but they are not exactly interchangeable in every structure.

Isara is the straightforward choice when the focused noun is the actual thing being closed, such as pinto, bintana, kurtina.

Sarhan uses a different verb pattern, and that often changes how the rest of the sentence is built and what noun becomes central.

So for a learner, the safest takeaway is:

• Isara ang kurtina = a standard way to say close the curtain
• Sarhan is a different pattern that you will learn as part of Filipino verb-focus system

By itself, kurtina is usually understood as curtain in the singular.

If you clearly want curtains, Filipino often uses mga:

ang mga kurtina

In this sentence, ang kurtina is most naturally read as the curtain. Context, of course, can always make the intended number clearer.